Helicopter Crash Rotations: Understanding Rotor Dynamics

Helicopters do not spin around when they crash, but they rotate rapidly due to dissymmetry of lift on the rotor disk caused by: rotor dissymmetry (main and tail, rotor RPM differences); wind (ground effect wind, translational lift); or wake from another helicopter/obstacles. As the craft descends rapidly under power into its vortex ring state the combination of forces on the fuselage will induce a roll moment. Without sufficient altitude to correct or counter the descent there's insufficient blade speed and lift to regain control of direction; hence, rapid, uncontrolled and possibly fatal rotational forces occur at ground strike.